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Adsorption properties at an air/water interface and interaction between a nonionic polymer ethyl-(hydroxyethyl) cellulose (EHEC) and sodium dodecyl sulfate (SDS) were studied by pendant drop tensiometry, enhanced by video-image digitization and computerized data handling. Three studies were performed over the concentration range 0-19 mM SDS with fixed concentrations of EHEC: 0.2% w/w, 12 ppm, and 2 ppm. Particular emphasis was placed on the investigation of the dynamics of surface tension in the presence and absence of SDS. This study demonstrates that the onset of complex (cluster) formation can be identified by surface-tension measurements even when a very hydrophobic polymer such as EHEC is part of the complex. Considerable synergism in surface activity is observed in the EHEC-SDS mixtures. Below the critical aggregation concentration (cac), the degree of surface-tension reduction was found to depend on the bulk concentration of surfactant molecules, as well as the number of polymer segments in actual contact with the surface at the time of measurement. At and above cac an extraordinary acceleration of polymer adsorption was observed following the addition of SDS. Considerable evidence regarding the presence of a polymer/surfactant complex at the air/water interface is discussed in the paper. The dynamics of surface tension, at and above cac, are explained in terms of the observations that charged clusters are bound along the polymer chain and cause an appreciable change of the polymer conformation. In agreement with the results, polyelectrolytic properties of the EHEC/DScomplex cause the complex to become firmly adsorbed to the air/water interface. The adsorption kinetics strongly suggested a progressive ordering of the polymer/surfactant aggregates within the monolayer, based on observations of the surface film after aging (17 h). The findings can be of practical use in applications where quick wetting and absorption are required, e.g., in injected, spread, or inhaled drug formulations. LA960976I

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The crystal structure of ammonium formate has been determined at room temperature from threedimensional single-crystal X-ray data. Two HCOONH4 units crystallize in a monoclinic unit cell of space group Pc and dimensions a=3.814, b=4.679, c=9.112A,, and B=91"17 °. The structure was refined by the method of least squares with allowance for anisotropic thermal motion, and gave a final discrepancy index of 0-067. The ammonium ion forms four hydrogen bonds N+-H • • • O-of length 2-81-2.89 A,, in an approximately tetrahedral arrangement. In the three-dimensional hydrogen-bonded network thus formed each oxygen atom accepts two hydrogen bonds. The dimensions of the formate ion are: bond lengths, C-O = 1-237 and 1-246 A,; bond angle, O-C-O = 126.3 °.